Offshore controlled-source electromagnetic (CSEM) geophysical surveys use man-made electric and magnetic sources to generate electromagnetic fields to excite the earth and deploy instruments/receivers in the ocean, on the seafloor and inside boreholes to measure electric and magnetic fields. FIG. 1 is a schematic diagram of such a survey, with electromagnetic source 11 connected by cable to a vessel and receivers 12 located in the ocean, and often on the seafloor 13. The measured fields are analyzed to investigate the sub-sea floor structures of the earth's interior. This technology has been applied not only in oceanic tectonic studies but also in offshore hydrocarbon and mineral exploration (A. D. Chave et al., in Electromagnetic Methods in Applied Geophysics 2, 931-966 Society of Exploration Geophysicists (1988); S. Constable and C. S. Cox, J. Geophs. Res. 101, 5519-5530 (1996); L. MacGregor et al., Geophy. J. Int., 146, 217-236 (2001); S. Ellingsrud et al., The Leading Edge, 972-982 (2002); T. Eidesmo et al., First Break 20.3, 144-152 (2002)).
The electromagnetic signals recorded by receivers consist of electromagnetic fields 21, 22 and 23 that travel through the earth 33, seawater 32, and air 31, respectively, as illustrated in FIG. 2. The signal 23 that travels partly through air is called an “air wave.” Offshore controlled-source electromagnetic geophysical surveys are normally operated at frequencies below 1.0 KHz. It is well known that, in this quasi-static frequency regime, penetration of electromagnetic waves into a medium varies inversely with both the frequency of the wave and the conductivity of the medium. This result follows from the theory of skin effect phenomena (J. A. Stratton, Electromagnetic Theory, page 504, MacGraw-Hill (1941)). Since the seawater is much more conductive than both air and the earth, electromagnetic signals through the seawater decay much faster than through the air and the earth. So, for source and receiver offsets longer than approximately twice the seawater depth, the recorded electromagnetic fields mainly come from through the air and the earth. However, only the signals traveling through the earth provide information of the sub-sea floor structures of the earth's interior. For deep sub-sea floor targets 34, electromagnetic is fields need to be generated at low frequencies to ensure that the transmitted electromagnetic signals 25 penetrate to the target depth. Unfortunately, for “shallow” water depth relative to the target depth and at low frequencies, the air wave signal may be dominant at receivers 12 with long offsets to the source 11 so that the target signal is hardly distinguishable. Obviously, conditions are best for CSEM prospecting when signal 25 dominates the combined effects of signals 21, 22 and 23.
Air wave interference is a problem when measurements are made in the frequency domain, i.e., when the source continues to transmit its signals while data are being collected at the receivers. The simplest source signal is a sinusoidal signal with a selected frequency. For operational efficiency, multiple frequencies can be transmitted at the same time in the form of a complex waveform, such as a square wave. A complement to the frequency domain CSEM is the time domain CSEM. In time domain CSEM, the source is turned on and then turned off after a desired wave form is transmitted (for example, a pulse, a boxcar, or a step function). The air wave may not be a problem in time domain CSEM because the air wave will be recorded at an earlier time, separated from target signals. However, advantages offered by frequency domain CSEM in more sophisticated modeling and inversion software, better understood results, and higher-quality data make frequency domain CSEM used more widely in geophysical surveys than time domain CSEM. As persons skilled in the art will understand, notwithstanding the preceding observations, all CSEM data are actually obtained in the time domain, i.e., they are collected by a recording device as a more or less continuous stream of numbers, with the independent variable being time. What distinguish frequency domain CSEM are the way the experiment is conducted (continuous source) and the methods used to analyze and interpret the data whereby the data are decomposed into individual frequency components, e.g., Fourier analysis.
The air wave effect can be easily illustrated using a simple one-dimensional (1D) layered model. As shown in FIG. 3, from top to bottom, the model consists of five layers: non-conductive air 31, seawater 32 (conductivity=3.0 Siemens/m, depth to be varied in examples below), mud rocks 33 (1.0 Siemen/m, 1.0 km thick), resistive reservoir layer 34 (0.01 Siemen/m, 100.0 m thick), and basement 35 (1.0 Siemen/m). If the resistive layer 34 is the target and is removed from this model, a new model results and may be defined as the background model of the original model. A unit horizontal electric dipole source 11 directed in the x-axis (HEDX) is towed in the direction of the x-axis and 50 m above the seafloor. A seafloor receiver 12 is located right below the mid point of the source tow line (not shown in FIG. 3).
FIGS. 4A-4C are graphs of the amplitude of the x-component of electric fields (Ex) vs. source-receiver separation in the x-direction for both this 1D model and its background model. The seawater depth is 5.0 km in FIG. 4A, 1.0 km in FIG. 4B, and 100 m in FIG. 4C. FIGS. 4D-4F show the corresponding “unwrapped” phase, for the same three seawater depths. Unwrapped phase is obtained by changing absolute jumps greater than π to their 2π complement. The curves of small circles represent data from the 1D model and the solid lines are from the background model. For the seawater depth of 5 km (FIGS. 4A and 4D), there is negligible air wave effect on data from both models for all source and receiver separations plotted in the figure.
Large separation between the 1D model's curves 41 and 43 and its background curves 42 and 44 indicates that the signal from the resistive layer buried 1.0 km below the seafloor is significant when the source-receiver separation is larger than ˜2 or 3 km. (The lack of separation between the model and background curves for small source-receiver spacing is due to the correspondingly low attenuation of the water path 22 and the seafloor path 21. Contribution from those two signals dominates the received signal for receivers with small offset (source-receiver separation), even with the target layer in the model.) When the seawater depth is decreased to 1.0 km (FIGS. 4B and 4E), the separation between these two curves shrinks significantly because of the air wave effect, i.e., the path 23 in FIG. 2 now travels through is much less water and consequently attenuation of the unwanted air wave is greatly diminished. This effect is magnified with increasing offset. At offsets longer than ˜6 or 7 km, the air wave effect dominates the received signal for the background model. This can be seen from the background curves 46 and 48 in FIGS. 4B and 4E, in particular the break in slope of the amplitude curve 46 around 6 km and the constant phase of the phase curve 48 beyond ˜7 km. However, no such features appear in the data curves 45 and 47 for the 1D model with the buried resistive layer because the signal from the buried resistive layer is still stronger than the air wave effect for this 1D model with 1.0 km water depth. This no longer holds when the seawater depth is 100 m, for which FIG. 4C shows that model data with and without the resistive reservoir layer are hardly distinguishable in amplitude for all offsets. (The significant departure between the two phase curves of FIG. 4F for offsets greater than ˜3 km is primarily an effect of the infinitely extended 1D model used rather than being due to signal from the target; FIG. 9B shows this effect essentially eliminated with a more realistic model.) Matters would be even worse for field data with noise. This implies that the air wave effect dominates the received signal even though the signal from the subseafloor target is strong. The results from this example clearly demonstrate the problem of the air wave effect.
Air wave contribution was investigated by Chave and Cox in their theoretical numerical model study for offshore CESM exploration with an horizontal electric dipole source (A. D. Chave and C. S. Cox, J. Geophys. Res. 87, 5327-5338 (1982)). Chave and Cox realized that the effect of seawater depth would be important at large source-receiver separations, low frequencies, or in relatively shallow water. They pointed out that the effect can be incorporated into the theory if both water depth and source location are accurately determined, but they did not disclose any method to compute the effect or remove it from CSEM data.
Eidesmo, et al., in the First Break article cited previously, not only described the features of the effect of the air wave on the amplitude and phase but also observed that the range at which the air wave dominates the is response, and information on seabed resistivity is lost, increases with decreasing frequency and water depth. The effect of the air wave can therefore be minimized by choosing appropriate transmission frequencies, and by targeting surveys on prospects in deep water and in which the target is at a relatively shallow depth below the seabed. However, the tactics of working outside of the air wave dominant range by carefully choosing transmission frequency and survey prospects cannot be used for prospects where the air wave effect is unavoidable for frequencies which can excite targets to generate recordable signals.
In a theoretical study of electromagnetic investigation of the sea floor using a vertical magnetic dipole (VMD), Coggon and Morrison concluded that, with a poorly conducting seabed, the total horizontal magnetic fields result from energy propagating in two main ways: through the bottom (the seabed) and through the air just above the sea surface. They also computed the air contribution and compared it with the actual effect of the sea/air interface alone to demonstrate that total out-of-phase magnetic field response is approximately the simple sum of air and bottom path contributions (J. H. Coggon and H. F. Morrison, Geophysics 35, 476-489 (1970)). However, this air wave contribution computation is performed primarily to verify the concept of two main energy propagating pathways, not to enhance the target signal by removing the air wave effect from the measured data as the invention does. The authors' conclusion about what use to make of their research is summed up in the quotation, “in practice it may often be desirable to operate with D/R above this minimum so that variations in sea depth can be neglected.” D is water depth and R is source-receiver separation. Thus, like Eidesmo et al., Coggon and Morrison teach to avoid conditions such as shallow water depth or large source-receiver separations that tend to make air waves a serious noise source in CSEM data.
Accordingly, there is a need for a reliable method for removing the air wave effect from frequency domain CSEM data in applications where such noise is unavoidable. The present invention satisfies this need.